Validate the port number on connection settings

[mmmries]

Error messages from gen_tcp are not very helpful. Should we consider validating some connection arguments before calling the low-level libraries?

I paired with the head of devops at my current job because we were seeing a confusing error message in our staging environment.

iex(spiff@10.0.1.49)5> cfg = v(4)
%{
  host: "nats.raw.svc.cluster.local",
  port: "4222",
  tls: true,
  token: "somerandomtoken"
}
iex(spiff@10.0.1.49)6> Gnat.init(cfg)
** (exit) an exception was raised:
** (FunctionClauseError) no function clause matching in :inet_tcp.getserv/1
(kernel 7.3) inet_tcp.erl:55: :inet_tcp.getserv("4222")
(kernel 7.3) gen_tcp.erl:181: :gen_tcp.connect1/4
    (kernel 7.3) gen_tcp.erl:165: :gen_tcp.connect/4
(gnat 1.2.0) lib/gnat/handshake.ex:9: Gnat.Handshake.connect/1
    (gnat 1.2.0) lib/gnat.ex:201: Gnat.init/1
(stdlib 3.14.1) erl_eval.erl:680: :erl_eval.do_apply/6
    (elixir 1.11.4) src/elixir.erl:280: :elixir.recur_eval/3
(elixir 1.11.4) src/elixir.erl:265: :elixir.eval_forms/3
    (iex 1.11.4) lib/iex/evaluator.ex:261: IEx.Evaluator.handle_eval/5
(iex 1.11.4) lib/iex/evaluator.ex:242: IEx.Evaluator.do_eval/3
    (iex 1.11.4) lib/iex/evaluator.ex:220: IEx.Evaluator.eval/3
    (iex 1.11.4) lib/iex/evaluator.ex:102: IEx.Evaluator.loop/1
    (iex 1.11.4) lib/iex/evaluator.ex:32: IEx.Evaluator.init/4
(stdlib 3.14.1) proc_lib.erl:226: :proc_lib.init_p_do_apply/3
    (kernel 7.3) gen_tcp.erl:170: :gen_tcp.connect/4
(gnat 1.2.0) lib/gnat/handshake.ex:9: Gnat.Handshake.connect/1
    (gnat 1.2.0) lib/gnat.ex:201: Gnat.init/1

The port was configured as a string (read from an environment variable), but it took some poking at to realize what the problem was.